TOPIC 5: ATOMIC STRUCTURE
learned early that matter is made up of small, indivisible particles.
Everything around us is made of extremely small particles. These
particles are either molecules or atoms. An atom is the smallest
indivisible particle of an element that takes part in a chemical change.
Atoms are the building blocks of matter. All solids, liquids and gases
are made of atoms fitted in different ways.
present day chemistry is built on the foundations of the Atomic Theory.
The idea that elements are made up of atoms is called the Atomic
Theory. An English chemist, John Dalton was the first to put forward the
Atomic Theory, which for most of the 19th century stated that atoms were hard, extremely small, indivisible and spherical particles like minute lead shots
Dalton Contribution to Atomic Structure
Explain Dalton contribution to atomic structure
Greek philosopher Democritus (460-370 BC) believed that matter was
indestructible and that it is made up of tiny particles called atoms.
Our modern understanding is based on the Atomic Theory which was put
forward by John Dalton in 1808. His theory re-introduced the ideas of
Democritus and other Greek philosophers who suggested that all matter
was infinitely divided into very small particles called atoms. These
ideas were not widely accepted at that time. They were only revived when
Dalton developed them further and experimental science was able to back
them up with practical observations.
Atomic Theory goes back to ancient Greeks, yet we always talk today
about Dalton’s Atomic Theory. There is a good reason for this. The
reason is that, while Greeks put forward the idea that atoms exist they
did nothing more. They left the idea vague and untested. Dalton changed
this vague imaging into a set of concrete suggestions about atoms which
could be tested by experiment. This change from vagueness to precision
and experimental test justifies his claim to the theory.
Dalton’s Atomic Theory contains the following main ideas:
- Matter is made up of small, indivisible particles called atoms.
- Atoms of the same element are all exactly alike in every way and have definite weights.
- Atoms are indestructible and they cannot be created.
- Atoms of different elements have different weights and posses different properties.
- Atoms of different elements combine in small whole numbers to form ‘compound atoms’.
The Modern Concept of Dalton’s Atomic Structure
Explain the modern concept of Dalton’s atomic structure
the theory, it is observed that each atom has its own mass and that
chemical combination takes place between atoms and not fractions of
made in the 20th century, however, showed that certain parts of the
theory must be modified. However, Dalton‟s Theory was one of the great
leaps of understanding of chemistry. It meant that we could explain many
Atomic Theory was the first step towards the formation of Modern Atomic
Theory. The Dalton’s Theory has been subjected to numerous
experimentations that have led to some modifications to the theory.
However, some ideas in his theory still hold strongly to date. Some
modifications to the theory include the following:
atom is no longer regarded as indivisible, or the smallest particle.
Particles smaller than the atom; electrons, protons and neutrons are now
known. However, the atom is still the smallest particle which can take
part in a chemical reaction.
- Atoms of the same element may not
be all alike. Some elements have atoms with different atomic masses e.g.
carbon 12 and carbon 14. These different atoms of the same element are
- In some few cases, atoms of different elements
may have the same atomic mass. Both argon and calcium have atomic mass
40. Such atoms are called isobars.
- “The compound atoms” of
Dalton are known as molecules. A molecule is the simplest particle of
matter which is capable of independent existence. Evidence is available
where atoms of different elements combine in large integers. An example
is in organic and silicon compounds.
- Atoms are no longer
regarded as indestructible. Radioactive atoms may get destroyed by
spontaneous decay or by atomic fission.The atom is therefore the
smallest particle of an element which is responsible for the chemical
properties of that element, and which takes part in a chemical reaction.